Radio station



June 7, i932. v. E. TROUANT 1,361,462

RADIO STATION Filed May 5, 1928 A BY N ATTNEY Patented .lune 7, 1932lUNITED STATES PATENT-orner.

VIBGIL E. TBOUANT,'OF WILKINSBURG, PENNSYLVANIA, ASSIGNOB T0WESTIHGHOUST ELECTRIC MANUFACTURING GUMPANY, A CORPORATION 0I"mNSYIavAN-t nemo STATION' appneaubn mea may a, 192s. smal le. $74,792.

This invention relates to radio broadcasting and particularly to thatmethod of broadcasting in which the modulation is effected by smallchanges in the frequency of the radiated wave.4

It is common practice to control the fre-- quency of the output of aradio broadcasting station by means of a master oscillator.- It hasrecently been found that a signal may be impressed upon the output bvarying the frequency of the master osci lator, the changes in frequencybeing so small that, al-

though the degree of response of the tuned circuit in the receiving setis altered by such frequency changes, the response is not sutilcientlyreduced thereby to causel the signal to be lost.

I have discovered that, when the modulation is produced by means offrequency changes, the number of amplifiers may be reduced and theamplification effected in each stage increased because it is no lon ernecessary to restrict the operation of t e amplifying tubes to thoseportions oftheir'characteristics which are substantially straight.

It is an object of my invention to take advantage of this circumstanceto obtain faithful reproduction of the signal by ampliers working overgreater parts of their characteristics than has heretofore beenpractical and thus to provide the necessary increase 1n.'

frequency and in amplitude between the mas- I ter oscillator and theantenna by a. minimum number of tubes.

It is a further object of my invention to provide for sending the samesignal at af plurality of different carrier-wave frequencies.

It is a further object of my invention to provide for the simultaneousbroadcastingl of two signals upon a single carrier frequency. p

It is a 'furtherV object of my invention to provide frequency-shiftmodulation and amplitude modulation of the same carrier wave 1n such away that the two lmodulations may both be received.

Other objects of my invention and details of the construction used maybe learned from the following description in connection with theaccompanying drawin in which the single figure 1s a dia am of t ecircuits and apparatus embodie vThe master oscillator :1 may be of anyvstandard or well known type. A master oscillator of the typecont-rolled by a piezoelectric crystal has been chosen for illustration.The crystal 2, in parallel with an inductance 3 and a C battery 4, isplaced, as usual, in the grid circuit of the master oscillator 1. Oneportlon of the plate circuit, including a condenser 5 and an inductor 6,is energized, 1n the usual way, from a portion of the B battery 7.

An adjustable portion of the inductor 6 is shunted rlan impedancecontrolled b the signal. s impedance includes a -tu or 'several tubes 10'and 11 in parallel. The

plates of the tubes 10 and 11 are connected together and to theadjustable contact 12 upon the inductor 6. The filaments, of the tubes10 and 11 are connected together and t tothe filament of themaster-oscillator tube 1, all of said filaments being supplied from acommon A battery 13 and adjusted by a common rheostat 14.

The grids of the tubes 10 and 11 are connected together and, through thesecondary 15 of a transformer 16, to the C battery 17 and, through thisbattery, to the filaments. The primary of the transformer 16 is suppliedwith slgnal-frequency current, als indicated by the microphone 20 andthe battery 21.

The masteroscillatorl is coupled to a frequency changer 24 through atuned coupling circuit, constitutin another portion of the plate circuitand incv uding an inductor 22 and a condenser 23 in parallel. Thecouplin -includes adjustable connections 25 and 26 rom the masteroscillator to the inductor 22.

The coupling between the connection 26 and the lower end of the inductor6 isthrough two condensers 27 and 28 in series. The condenser 27- servesto conduct the high-frequency current in the inductor 22 around thewhole of the batt-ery 7 which sup lies the needed ,to prevent tubes 10and 11 from being\ merely rectiiers.

The frequency chan er 24may be of any desired or well known orm. Formost of the objects of this invention, it is necessary that thefrequency of the output of the frequency changer shall be a constantmultiple of that of the inputthereof. The output circuit of thisfrequency changer is connected in parallel to the input circuits ofseveral frequency changers and to a power amplifier 30. rlhe output ofthe frequency multiplier 24 is 1mpressed upon the grid and the filamentof the tube 30. The oscillations delivered by the tube 30, are,therefore, of the frequency supplied by the output of the frequencymultiplier 24.

A resistor 29 may be connected'across the line from the frequencymultipilier 24 to the tube 30, to serve as a grid-lea When the line islong enough to make the consideration of surges necessary, theresistance of resistor 29 should equal the surge impedance of the line,to minimize reiection. If desired, a grid condenser may be included inthe line to provide the grid bias for the tube 30.

The tuned circuit, including an inductor 31 and a condenser 32 in theoutput of the tube 30 is adjusted to the frequency of the output of themultiplier 24. The frequency in the circuit 31-32 is, consequently, aconstant multiple of the frequency delivered by the master oscillator 1.

rEhe output circuit of the frequency multiplier- 24 is also connected tothe input circuit of the frequencymultiplier 33. The output of thefrequency multiplier 33 is connected to the input of the power amplifier34. The power amplifier, therefore, delivers energy at a frequency whichis a constant multiple of the frequency generated by the masteroscillator 1 but a'diiferent multipleI thereof from the frequency in thecircuit 31-32.

Similarly, the output of the frequency multiplier 24 is connected totheinput of a frequency multiplier 35 having a differentfrequency-multiplying factor from that of the frequency changer 33. Theoutput of the frequency changer 35 is delivered to the power amplifier36. The output of the power ampliiier 36 is thus a constant multiple ofthe frequency of the master oscillator 1 but the multiple is differentfrom that for the power amplifier 34 and also different from that forthe amplifier 30.

Energy for the power amplifier 30 is supplied from a direct-currentsource 40, through an audio-frequency choke coil 41. Between the chokecoil 41 and the plate of the tube 30, a radio-frequency choke coil 42 isinserted. The audio-frequency choke coil 41 is also connected to theplate of the tube 43.

The grid of the tube 43 is influenced by any suitable signalling device.A microphone 44, with the accompanying battery 45 and transformer 46,similar to the corresponding elements associated with the micro hone 20,is illustrative of the signalling evice. The tube 43 is thus associatedwith the tube 30 in a way closely similar to the constant-currentmodulation systems already known.

The tube 30 and each of the power amplilfiers are coupled to respectiveantennae 52, 54

pedance in shunt to a portion of the inductor i,

6 and correspondingly alter the frequency of the current generated bythe master of oscillator 1. This frequency is multiplied by thefrequency multipliers, and the currents impressed upon the severalsignalling circuits have respective frequencies equal to the frequencyat the master oscillator multiplied by the product of the multiplyingfactors of the frequency-increasing devices between the masteroscillator and the respective signalling circuits.

Since the multiplying factor of each frequency changer or eachcombination of frequency changers is different, the radiations deliveredby the several antennae will be of dierent frequencies. If, for example,the frequency multiplier 24 delivers the third hormoni'c of themaster-oscillator frequency, the frequency multiplier 33 delivers theseventhharmonic of the frequency received by it, and the frequency'multiplier 35 delivers the fifth harmonic of the frequency received byit, the antenna 52 will send a frequency three times that of the masteroscillator; the antenna 54 will send a frequency twenty-one times thatof the master oscillator and the antenna 56 will send a frequencyfifteen times that of the master oscillator.

These numbers are chosen for illustration only. ln usual practice, themultiplying factors are ordinarily much larger than this, in order thatthe frequencies delivered to the antennae may be large enough to bereadily radiated even when the master oscillator delivers a frequencywhich is not very high.

The signal impressed upon the microphone 2() causes changes in thefrequency of the master oscillator and changes, which are larger inabsolute value although not larger in percentage value, occur in each ofthe frequencies radiated from the several antennae.

Practically all receiving sets have a tuned circuit. The response insaid circuit is of di fferent intensity for different frequencies. rlfhecharacteristic curve, showing the intensity of response in such circuitthroughout a rangepf frequencies, comprises a peak, the summit of whichcorresponds to the frequencylto which the circuit is tuned, andlowfrequency produce proportional changes in intensity of response inthe tuned circuit.

If, therefore, the receiving circuit be so tuned that the averagefrequency of the received wave corresponds to a point in the midst ofone of the straight parts of the curve, the changes in intensity beingproportional to the frequency changes impressed upon the carrier wave bythe frequency-shift modulation, will reproduce the signal. So long,therefore, as the changes in frequency do not extend beyond the straightpart of the curve, the signal impressed upon the microphone 20 will befaithfullyreproduced in the receiving set.

By tuning the receiving set, as described, so that the carrier-frequencyfrom one antenna, say 54, corresponds to a point slightly to one side ofthe resonance peak, the signals are received by frequency-shiftmodulation over one channel.

Another receiving set tuned to the carrier frequency radiated by theantenna 56 would receive the signal in the same way, but' over adifferent channel.

It frequently happens that atmospheric conditions cause a greater fadingat one carrier frequency than at another. If two .or more frequenciesare conveying the same signal and one fades, the operator of thereceiving set may obtain the signalover the other.

Again, by cooperation between the operator of the receiving set and theoperator of the sending set, a prearrangement may be effected by whichthe frequency upon which the signal is to be sent is known only to thesender and receiver and other listeners will be prevented, or at leasthindered, from receiving the message.

Obviously, any number of different frequencies may thus be used forsending and any predetermined change from one to another of thesefrequencies may be effected instantly by switching on oroff any one ormore of the frequency multipliers.

Since thechange to be delivered to the signalling circuit in order tocommunicate the signal is a change not of amplitude but of frequency, itis unnecessary that the operating characteristics of the tubes used inlthe several amplifiers shall be linear. This applies not only to thepower amplifier but to any amplifying tubes used in theIfrequencymultiplying devices. The total number of amplifying tubesneeded in order to modulate a powerful carrier wave is thus reduced,because each amplifier may be'used to produce a much reateramplification than is possible when t e output current must beproportional to the input.

Ampliers intended to respond to amplitude modulation need aconstant-bias device to keep the average grid potential` near the centerof the straight part of the tube operating characteristic. yAmplifiersintended for frequency-shift modulation are not thus restricted and suchfixed grid-biasing devices are not needed with them. The amplifier atany relaying station may be a duplicate of f that at the originalsending station when frequency-shift modulation is used.

v 'The receiving set may, if adjusted to the carrier frequency deliveredby the antenna 52, obtain the signal from the micro hone 20 in the sameway as from either of t e other antennae. In order to obtain the signalcorresponding to the frequency-shift modulation, the receiving set mustbe so adjusted that the average frequency from the antenna 52 is notvery near the peak of the characteristic curve of the resonant circuitin the receiving set. l

If the adjustment is so chosen that the average or carrier frequencyfalls near the lower end of the straight part of the characteristiccurve, changes in amplitude of Vthe waves radiated from the antenna 52will cause but little difference in the` intensity of the signalreceived.

' Even this small change of intensity may be sufficient to interferewith satisfactory reception of voice or music over the frequencyshiftapparatus, but the reception of code signals is easy with thisadjustment. On the other hand, changes in frequency will produce nearlyas great a change in lntensity as if the adjustment were such as tobring the `average frequency at the middle of the side of the peak. Withthis adjustment, therefore, signals, or at least, code signals,impressed upon the microphone 20 may be received even in the presence ofthe signal im-v pressed upon the microphone 44.

If, instead of an adjustment which brings the average frequency upon theside of the peak, the adjustment be so made that the average frequencyis at the crest of the peak, changes in frequency Awill make but a smallchange in the resulting intensity in the receiving set, while changes inamplitude 'will, with this adjustment, make maximum changes of intensityin the receiving set.

A receiving set, therefore, tunedapproximately to the carrier frequencyof the radiation from the antenna 52 may, by adjustment v of the tuning,be made to reproduce either the from the microl hone 44. In this way, itisy 'possible to sen two signals simultaneously over the same wavelength.

Moreover, when the adjustmentis made to obtain the signal correspondingto frequenc -shift modulation, it is possible to minimize t e signalcorresponding to amplitude modulation and, when the si al correspondingto amplitude modulation 1s desired, that correspondin to fr uencymodulation may be practica ly comp etely eliminated. y

When the tone impressed upon the microphone is of constant pitch, as isfrequently the casein code si allmg, a band-pass filter may be added int e receiving setto prevent interference with the .frequency-shiftsignal by the amplitude modulation. When such a set is used to receivethe signal byamplitude modulation, the filter should be switched out ofuse. It is thus possible to obtain either si al without interferencefrom the other.

monica circuits a common master oscillator there-v said sendingcircuits.

In testimony whereof, I have hereunto subscribed my name this 27th dayof A ril 1928.

VIRGIL E. TRO ANT.

en using code signals of constant pitch,

an audio-frequency generator and a key may be substituted for themicrophone 20 and battery 2l. Code signals may also be sent, if thereceiving set is of the heterodyne type; by charging the direct-currentIbias of tu s 10 and 11 by means of a key. Again, the invention isuseful in systems, like television, where synchronizing must be carriedon at the same time as other signalling. For example, theamplitudeiinodulation may correspond to light variation and a shift offrequency at regular intervals, corresponding to the keying of tubes 10and 11 may correspond to synchronizing. The latter' will not interferewith the amplitude modulation or with the reception thereof and, becauseof the regularity of the frequency shift, the amplitude modulation willnot interfere with the reception of the synchronizing signal and theconsequent change of heterodyne beat note. Y

Many modifications of the circuits andiapparatus and many otherapplications of my invention will readily occur to those lskilled in theart. That the above description mentions only a few of these is not tobe construed as a limitation. No limitation is intended, except such asis required by the prior art or stated in the claims.

I claim as my invention z' l1. In combination, a plurality of sending Icircuits, a common master oscillator therefor, diferentfrequency-multiplying means between said master oscillator and saidsending circuits, whereby the frequencies impressed on said circuits,respectively, are different multiples of the frequency of the masterosrcillator, vmeans for varying the master-oscillatorxflequency inaccordance with a signal and signal-,controlled means for varying theamplitude in\atleast one of said sending circuits. y

2. In combination, a plurality of sending

